Guarda questo....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function contaRighe(aTextarea) {
nblines = 0;
re = /\n/g;
value = aTextarea.value;
arr = value.match(re);
if (arr != null) {
nblines = arr.length;
} // if (arr != null)
document.getElementById('d').innerHTML = nblines + 1;
return;
} // function contaRighe(aTextarea)
//-->
</script>
</head>
<body>
<textarea id="t" rows="4" cols="50" onkeypress="contaRighe(this)" ></textarea>
<div id="d"></div>
</body>
</html>